In general, existing methods of screening liquids which carry solids in suspension attempt to remove some or all of those solids using fixed or reciprocating, perforated or bars screens situated within the liquid flow. These screens must be routinely cleared to remove the build-up of solid material or screenings from the face of the screen in order to maintain adequate flow of liquid through the screen.
Typically, this clearing operation is carried out by one or more of a number of known mechanical methods, such as the action of rakes, scrapers or brushes on the screen. These methods necessarily require expensive equipment such as motors, gearing and drive systems as well as brushes or scrapers which require regular servicing and/or replacement, with consequent implications for the cost and reliability of the operation of such systems.
Such equipment also invariably consumes substantial quantities of electrical power. Not only is this expensive, but the times when frequent blockage of sewerage systems is most likely, e.g. during storms, can also be the times when the electricity supplies are most vulnerable to interruption. In any event, there are particular problems associated with screening to remove solids and floatables from systems which handle both rainwater and sewage. During heavy rainfall, the collected rainwater flow can easily be one or two orders of magnitude greater than the sewage flow which can result in the system exceeding its capacity and overflowing.